Conventionally, devices, such as radar apparatuses and fish finders, for transmitting and receiving a sound and/or an electromagnetic wave perform various kinds of processing to remove interference caused by another device(s). For example, JP2003-322678A discloses a method that is performed in the fish finder, in which interference is detected based on whether a ratio of an amplitude of a reception signal from a measurement (current measurement) and an amplitude of a reception signal from a previous measurement is below a predetermined value.
The interference signal is unlikely to be detected continuously for a plurality of times at the same level and the same depth. The method disclosed in JP2003-322678A includes determining that the current reception signal is interference if the ratio of the amplitudes is a predetermined value or higher (i.e., if an intensity difference between the reception signals is large) and displaying it on a display using the previous reception signal.
However, because the method disclosed in JP2003-322678A determines as interference as long as the intensity difference is large, there is a possibility that a reception signal that is a normal echo of other than interference (a target object such as fish) may be determined as interference. For example, when the target object moves at a high speed in a depth direction, the echo signals of the current measurement and the previous measurement from the same target object may be detected at depths far apart from each other. In this case, when comparing the intensities of the current reception signal and the previous reception signal at the same depth, the intensity difference is determined to be large and thus the current reception signal is determined as the interference. Therefore, an echo image that should normally be displayed may become smaller in size or not displayed at all.